1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) which utilizes opto-electric anisotropy of liquid crystal, and more particularly to a liquid crystal display which achieves an improved response speed and transmittance.
2. Description of the Related Art
LCDs are compact, thin, and low power consumption devices and have been developed for practical use in the field of office automation (OA) and audio-visual (AV) equipment. In particular, active matrix type LCDs which utilize thin film transistors (TFTs) as switching elements are theoretically capable of static actuation at a duty ratio of 100% in a multiplexing manner, and have been used in large screen and high resolution type animation displays.
TFTs are field effect transistors arranged in a matrix on a substrate and connected to individual pixel electrodes which form one side of pixel capacitors with a dielectric layer made of liquid crystal. In a TFT matrix, TFTs located on a same row are simultaneously turned on/off by a given gate line, and each TFT of that row receives a pixel signal voltage from a given drain line. A display voltage is accumulated in the pixel capacitors corresponding to the on-state TFTs and designated by rows and columns. The pixel electrodes and the TFTs are formed on the same substrate, while a common electrode acting as the other side of the pixel capacitors is formed almost entirely on the surface of the second substrate opposite to the first substrate across the liquid crystal layer. That is, the display pixels (i.e., pixels) are defined by partitioning the liquid crystal and the common electrode by pixel electrodes. The voltage accumulated in the pixel capacitors is held insulated by an off-state resistance of the TFTs for one field period or one frame period until the TFTs are turned on again. The liquid crystal is opto-electrically anisotropic, and its transmittance is controlled based on the voltage applied to respective pixel capacitors. The transmittance of each display pixel is independently controlled, so that individual pixels are observed bright or dark and recognized collectively as a display image by human eyes.
Initial orientation of the liquid crystal is determined by an orientation film disposed at the interface between the liquid crystal and each substrate. For example, a twisted nematic (TN) type LCD uses the liquid crystal in nematic phase which has positive dielectric anisotropy and whose alignment vectors are twisted 90 degrees between the substrates. Typically, a polarizing plate is provided on the outside of each substrate, and, in a TN type, the polarizing axis of each polarizing plate coincides with the orientation of the liquid crystal located in the vicinity of the corresponding substrate. When no voltage is applied, linearly polarized light passes through one polarizing plate, turns its direction in the liquid crystal layer along the twisted alignment of the liquid crystal, and exits from the other polarizing plate, resulting in a “white” display. When the voltage is applied to the pixel capacitors, an electric field is created within the liquid crystal layer and the orientation of the liquid crystal changes its orientation to be parallel to the direction of the applied electric field because of its dielectric anisotropy. As a result, the twisted alignment is collapsed and the incoming, linearly polarized light turns less frequently in the liquid crystal. Consequently, the amount of light ejecting from the other polarizing plate is reduced and the display gradually becomes black. This is known as a normally white mode which is widely applied in the field of TN cells, in which the display is white when no voltage is applied and changes to “black” upon application of a voltage.
FIGS. 1 and 2 show a unit pixel structure of a conventional liquid crystal display, wherein FIG. 1 is a plan view and FIG. 2 is a sectional view along line G-G of FIG. 1. A gate electrode 101 made of a metal, such as Cr, Ta, or Mo, is formed on a substrate 100, and a gate insulating film 102 made of, e.g., SiNx and/or SiO2 is formed to cover the gate electrode 101. The gate insulating film 102 is covered with a p-Si film 103 in which an implantation stopper 104 which is made of SiO2 or the like and patterned into the shape of the gate electrode 101 is used to form a lightly doped region (LD) having a low concentration (N−) of impurities, such as P or As, and source and drain regions (S, D) having a high concentration (N+) of the same impurities located outside the LD region. A region located immediately below the implantation stopper 104 is an intrinsic layer which includes substantially no impurities and acts as a channel region (CH). The p-Si 103 is covered with an interlayer insulating film 105 made of SiNx or the like. A source electrode 106 and a drain electrode 107, both made of a material such as Al, Mo, or the like, are formed on the interlayer insulating film 105, each electrode being connected to the source region S and the drain region D, respectively, via a contact hole CT1 formed in the interlayer insulating film 105. The entire surface of the thus formed TFT is covered with a planarization insulating film 108 made of SOG (spin on glass), BPSG (boro-phospho silicate glass), acrylic resin, or the like. A pixel electrode 109 made of a transparent conductive film such as ITO (indium tin oxide) is formed on the planarization insulating film 108 for actuating the liquid crystal, and is connected to the source electrode 106 via a contact hole CT2 formed in the planarization insulating film 108.
An orientation film 120 formed by a high molecular film, such as polyimide, is disposed on the entire surface on the above elements and is subjected to a rubbing treatment to control the initial orientation of the liquid crystal. Meanwhile, a common electrode 131 made of ITO is formed on the entire surface of another glass substrate 130 arranged opposite to the substrate 100 across a liquid crystal layer. The common electrode 131 is covered with an orientation film 133 made of polyimide or the like and is subjected to rubbing.
Here, a DAP (deformation of vertically aligned phase) type LCD is shown which uses a nematic phase liquid crystal 140 having negative dielectric anisotropy and vertical orientation films as the orientation films 120 and 133. The DAP type LCD is one of the electrically controlled birefringence (ECB) type LCDs which use a difference of refractive indices of longer and shorter axes of a liquid crystal molecule, so-called a birefringence, to control transmittance. In the DAP type LCD, upon application of a voltage, an incoming light transmitting through one of orthogonally placed polarization plates enters the liquid crystal layer as a linearly polarized light, and is birefracted in the liquid crystal to become an elliptically polarized light. Then, retardation, which is the difference in phase velocities of ordinary and extraordinary ray components in the liquid crystal, is controlled according to the intensity of the electric field in the liquid crystal layer to allow the light to be emitted from the other polarization plate at a desired transmittance. In this case, the display is in a normally black mode, since the display is black when no voltage is applied and changes to white upon application of an appropriate voltage.
As described above, the liquid crystal display displays an image at an intended transmittance or color phase by applying a desired voltage to the liquid crystals sealed between a pair of substrates having predetermined electrodes formed thereon and by controlling a turning route or a birefringence of light in the liquid crystal. Specifically, the retardation is controlled by changing the orientation of the liquid crystal, to thereby adjust the light intensity of the transmitted light in the TN mode, while allowing the separation of color phases in the ECB mode by controlling a spectroscopic intensity depending on wavelength. Since the retardation depends on the angle between the longer axis of the liquid crystal molecule and the orientation of the electric field, the retardation still changes relative to the viewer's observation angle, i.e., a viewing angle, even when such an angle is linearly controlled by the adjustment of the electric field intensity. Thus, as the viewing angle changes, the light intensity or the color phase of the transmitted light also changes, causing a so-called viewing angle dependency problem.
Problems of decreased transmittance and slower response speed also remain.